Laser sources are used in image forming apparatuses such as digital copiers and laser printers that produce images using electrophotography. In an electrophotographic process, an optical scanning unit having a laser source emits a modulated laser beam according to image data. The laser beam is projected onto a motor-driven polygon mirror rotating at a constant speed and reflected to scan a photoconductor surface line by line to obtain an electrostatic latent image. The latent image is subjected to subsequent development and transfer processes, where a visible image is developed with a developer, such as toner, and transferred to a recording sheet to obtain a printed output.
Typically, a laser source used in an image forming apparatus includes a laser diode (LD) provided with a driver circuit. The LD and the driver circuit are mounted together on an LD driver board that derives a substantially constant supply voltage from a direct current (DC) power supply, which may be used in common with a load circuit included in the image forming apparatus. Such a supply voltage can suffer fluctuations during laser emission when the load circuit consumes a large amount of power fluctuating over time, and these voltage fluctuations may occur in different degrees and/or manners depending on operations performed by the load circuit.
Referring to FIGS. 1A and 1B, charts showing the supply voltage for a non-operating state (FIG. 1A) and an operating state (FIG. 1B) of the load circuit are described.
As shown in FIG. 1A, the supply voltage remains constant at Vc when the load circuit is in the non-operating state. During laser emission, the load circuit carries out a given function to consume a large amount of power derived from the common DC power supply, causing fluctuations in the supply voltage. As shown in FIG. 1B, the supply voltage periodically oscillates with constant amplitude “A” and period “B” when the load circuit is in the operating state.
Such fluctuations in voltage applied to an LD driver board may result in defects in laser emission, leading to defective images produced through the laser emission. For example, voltage oscillations like those shown in FIG. 1B may cause inconsistencies in density of a produced image, commonly referred to as “banding”.
Referring to FIG. 2, an example of output graphics produced with a banding defect is described. As shown in FIG. 2, the graphic output has density inconsistencies that appear as a repeating stripe pattern. Each stripe of the banding pattern has a varying density in a constant width “b” resulting from the oscillating voltage, where the amplitude and period of the voltage oscillations correspond to the density range and width of the repeating stripes.
Several attempts have been made to remove voltage fluctuations and achieve stable laser operation in an electrographic process. One approach is to use a filter such as a capacitor that eliminates selected frequencies from a voltage supplied to a load circuit. This approach may be applied to an image forming apparatus having an optical reading unit and scanning unit which operate asynchronously using a common power supply. By inserting a filter in a power supply line of the reading unit, undesired frequencies or variations in a voltage applied to the reading unit can be eliminated to prevent fluctuations in a voltage supplied to the scanning unit.
A drawback of such a filter-based approach is that it requires accurate calculation to specify a filter suitable for a particular load circuit. In addition, a product having a plurality of load circuits powered from a common power supply each requiring a dedicated filter may be costly to manufacture. Hence, there is still a need for a system capable of effectively suppressing voltage fluctuations at low cost that can be applied to an optical scanning device to achieve stable laser operation in an electrophotographic process.